1. Field of the Invention
This invention relates to a floating spar for supporting a production platform, and more particularly to such a floating spar for supporting production risers extending from subsea manifolds to the production platform in deep water offshore wells.
2. Description of the Prior Art
Oil and gas production spars currently utilize a number of subsea wells placed a given lateral distance on the sea floor and connected to surface facilities via individual risers where a Christmas tree is attached for well control. Wells for deepwater typically are very heavy given their extended length and in some cases multiple barriers where multiple concentric casing riser joints exist. Since a production spar is a floating vessel, each riser must be vertically tensioned to maintain its structural integrity. Hydraulic piston assemblies, electro-mechanical devices, and dashpots are some of the mechanisms used to maintain a constant tension while the spar is heaving or moving laterally (due to the ocean environmental forces). Buoyancy devices attached to riser strings have also been used to allow the risers to free stand independently of the spar""s hull. This method is the most advantageous with respect to the spar since the tension created by the buoyancy devices are not transferred to the spar hull, thereby freeing up the displacement of the spar""s hull to support the weight of the spar and the facilities placed on top.
The drawback to this method is size. To make an offshore production spar economically viable, several wells must be tied back to the surface facility, each requiring a certain amount of space in the center of the spar for the riser and its buoyancy devices. As water depth increases, riser weight increases. As riser weight increases, space for buoyancy to hold up the riser increases. As the space increases, so does the spar""s hull diameter to accommodate the need for added space. If the spar""s hull is larger, it is more costly to build and install, requiring more wells. Therefore a spar may reach an economic limit, simply because the water depth and number of wells create a spar hull so large as to make it uneconomical. Another aspect that may increase riser weight or size is the concept of xe2x80x9cbarriersxe2x80x9d. If a well""s fluid control devices (tree and manifolds) are at the surface, there may be a requirement for extra conduits in the riser design for both structural protection and pressure containment. Added conduits will increase both size and weight to the riser.
U.S. Pat. No. 5,706,897 dated Jan. 13, 1998 is directed to a floating spar which is a deep-draft floating caisson of a hollow cylindrical construction and utilized primarily for deep water offshore well operations at depths of 2,000 feet or more. The floating spar is anchored by mooring lines to the sea floor and may extend seven hundred feet, for example, below the surface of the water. The spar or caisson shown in the ""897 patent is directed primarily to a caisson for drilling risers for supporting a high pressure drilling riser and a low pressure drilling riser extending from a subsea wellhead. FIGS. 9 and 10, however, are directed to production risers in which a subsea tree is added to provide a mechanical safety barrier at the sea floor. Above the subsea tree is the vertical riser extending to a production manifold at the surface. An additional surface tree is provided for fluid control purposes. Thus, a production riser extends from each subsea wellhead to the surface location via a subsea tree, riser conduit, surface tree, and surface manifold.
The utilization of individual production risers extending from each subsea wellhead through the spar to a surface manifold and surface tree results in a substantial weight exerted on the spar particularly when multiple subsea wellheads, such as ten or more, are being utilized for product supply. Also, a substantial space within the spar or caisson is required for the multiple lines extending through the space to the surface platform or deck. Floatation tanks within the spar are utilized for tensioning the risers. In some instances, the risers and wellhead connector are deployed and recovered through the internal diameter of the buoys. The buoys must therefore be sized to permit the passage of the large diameter wellhead connector which normally controls the internal diameter of the spar and contributes to the overall size of the spar.
It is desired that a spar be of a minimal size and weight for minimizing costs and simplifying construction, installation and operation.
The present invention is directed to an offshore production system utilizing a spar or caisson anchored to the sea floor by mooring lines and supporting a production platform above the sea level. A plurality of subsea wellheads each has a subsea tree mounted thereon with a removable tree cap to permit access to the subsea tree and subsea wellhead. Production conduits from the annulus and production bores of each subsea tree extend to either: a production riser to the spar or a subsea manifold which receives conduits from multiple subsea trees, such as five or ten subsea trees, for example. Subsea manifolds are normally provided, particularly when a plurality of the subsea wells are located nearby each other to reduce the number of conduits extending to a surface location. Production risers from subsea trees and/or manifolds extend from the sea floor through the spar to the production platform on top of the spar. Also, test lines and umbilical lines may extend from the subsea trees and manifolds through the spar to the production platform for flow control, test or maintenance work. The production risers from the subsea tree and manifolds may be flexible cables or vertical centenary risers and formed of various materials.
To intervene or provide access to the subsea tree, such as the tubing string, the spar may be positioned over the designated well with the intervention riser system over the tree. The tree cap is then removed and the intervention system is then landed and locked onto the top of the tree thereby permitting intervention in the well. To minimize intervention hardware weight and the number of trips that equipment has to travel between the surface and the sea floor, the subsea trees may utilize a light weight tree cap which may be deployed and recovered by a remotely operated vehicle (ROV).
Utilizing subsea technology, the costs of deepwater spars are reduced by reducing the number of risers between the sea floor and the spar. Instead of individual risers for each well, the wells are completed in a standard subsea configuration which are subsequently sent to the surface individually via a light weight minimal barrier riser, or co-mingled together via manifolding on the sea floor and sent to the surface by a single larger bore riser to the spar facility. The production riser(s) may be vertically supported in the same manner as individual well risers. The production riser itself may be larger in diameter than the individual well riser, requiring bigger buoyancy to support its weight. Other risers for pipeline pigging, well testing, and control (electrical/hydraulic line) cables to operate the subsea wells may also be needed, but the overall number of suspended conduits from the spar is drastically reduced for the same number of wells. The fewer number of conduits required results in a smaller space and spar hull size requirement; leading to lower spar hull fabrication costs. Subsea multi-well technology also does not limit the number of wells needed, nor the structural and geometric problems of a riser associated with the lateral reach out to outlying wells. In addition, single subsea wells with a subsea tree leading to a production pipeline/riser conduit act as both the safety barrier and flow control are a simpler design and a more cost effective approach to the subsea safety tree and surface tree on either end of the spar riser configuration.
The reduced area for risers also lets the spar better utilize its deck space and displacement capacity for drilling and workover derricks, subsea risers and subsea blowout preventers. With fewer risers, the spar may move about on its anchor mooring spread to position itself over any well for subsea drilling completion or workover operations permitting tubing intervention into individual subsea wells.
It is an object of this invention to provide a deep-draft floating spar of minimum size and weight for supporting production risers extending from subsea manifolds to a production platform on the spar.
A further object of this invention is to provide such a subsea production system utilizing subsea trees which have a removable tree cap for intervention and access to the subsea well without necessarily going through the production riser. Small intervention well control hardware can be run and suspended from the spar for periodic maintenance and workovers.
Another object of the invention is the provision of such a spar subsea production system in which subsea trees have production pipelines extending to subsea manifolds which, in turn, have production risers extending from the manifolds through the spar to the production platform thereby eliminating surface trees and minimizing any surface manifolds for the production platform.
Other objects, features, and advantages of the invention will be more apparent from the following specification and drawings.